In medicine, a plethysmograph is an instrument that measures physiological parameters, such as variations in the size of an organ or body part, through an analysis of the blood passing through or present in the targeted body part, or a depiction of these variations. An oximeter is an instrument that determines the oxygen saturation of the blood. One common type of oximeter is a pulse oximeter, which determines oxygen saturation by analysis of an optically sensed plethysmograph.
A pulse oximeter is a medical device that indirectly measures the oxygen saturation of a patient's blood (as opposed to measuring oxygen saturation directly by analyzing a blood sample taken from the patient) and changes in blood volume in the skin. Ancillary to the blood oxygen saturation measurement, pulse oximeters may also be used to measure the pulse rate of the patient.
A pulse oximeter may include a light sensor that is placed at a site on a patient, usually a fingertip, toe, forehead or earlobe, or in the case of a neonate, across a foot. Light, which may be produced by a light source integrated into the pulse oximeter, containing both red and infrared wavelengths is directed onto the skin of the patient and the light that passes through the skin is detected by the sensor. The intensity of light in each wavelength is measured by the sensor over time. The graph of light intensity versus time is referred to as the photoplethysmogram (PPG) or, more commonly, simply as the “pleth”. From the waveform of the PPG, it is possible to identify the pulse rate of the patient and when each individual pulse occurs. In addition, by comparing the intensities of two wavelengths at different points in the pulse cycle, it is possible to estimate the blood oxygen saturation of hemoglobin in arterial blood. This relies on the observation that highly oxygenated blood will absorb relatively less red light and more infrared light than blood with lower oxygen saturation.
Many SpO2 sensors are constructed with a male connector having pins molded in plastic into a shape that can be fitted into a complementary female connector constructed with openings for receiving the pins from the male connector. The female connector socket is typically located on the main housing of the oximeter. The standard RS 232 connector is one common type of connector used.
As with many medical devices, SpO2 sensors are designed for single patient use in order to prevent the spread of disease between patients as well as to ensure that the sensor will operate to the exacting specifications of a medical device. However, whether unwittingly or not, it is not uncommon for oximetry sensors to be reused for different patients. Regardless of the reason, this reuse is not desirable.